The present invention relates to an optical recording medium, and more in detail, it relates to an optical information recording medium for performing recording/reading-out and erasure of information in both of grooves and an inter-groove portions (lands) of a substrate by irradiation of a laser beam.
Along with increasing amount of information in recent years, a recording medium capable of recording and reading-out a great amount of data at a high density and at a high speed has been demanded, and an optical disk is expected as a medium just suitable to such an application use.
Optical discs include a write-once-type disk capable of recording only for once and a rewritable-type disk capable of recording and erasure over and over.
As the rewritable-type optical disk, there can be mentioned a magneto-optical recording medium utilizing an magneto-optical effect and a phase-change medium utilizing the change of reflectance along with reversible change between amorphous and crystal states.
The phase-change medium has a merit capable of recording/erasure by only modulating the power of a laser beam without requiring an external magnetic field, and capable of miniaturizing of a recording/reading-out device.
Further it has also a merit capable of obtaining a high density recording medium by a shorter wavelength with no particular alteration of materials from existent medium predominant at present capable of recording/erasure at a wavelength of about 800 nm.
As the material for the recording layer of such a phase-change medium, a thin film of chalcogenic alloy is often used. There can be mentioned, for example, GeTe-based materials, GeTeSb-based materials, InSbTe-based materials and GeSnTe-based materials.
Generally, in a rewritable phase-change recording medium, recording is performed by forming an amorphous bit from a crystallized state in a unrecorded/erased state. The amorphous bit is formed by heating the recording layer to a temperature higher than the melting point followed by quenching. In this case, a dielectric layer disposed in contact with the recording layer serves as a heatsink layer for obtaining a sufficiently overcooled state, and a protective layer for suppressing an ablation.
On the other hand, erasure (crystallization) is performed by heating the recording layer to a temperature higher than the crystallizing temperature and lower than the melting point of the recording layer.
In this case, the dielectric layer serves as a heat accumulating layer for keeping the temperature of the recording layer at a high temperature till crystallization is completed.
Generally in a writable phase-change recording medium, a laser beam of two different power levels is used for attaining different crystal states.
The recording film is selected from view points that the film can easily take a crystallized state or an amorphous state moderately, has a large difference of reflectance between the crystallized state and the amorphous state, and shows small volume change due to phase-change, or the like.
The material for the protective layer is selected from view points, for example, having optical transparency to a laser beam, appropriate refractive index, high melting point, softening point and decomposition point, ease of preparation and appropriate heat conductivity.
In a phase-change-type medium capable of 1-beam overwriting, the erasing and rewriting steps can be performed only by the intensity modulation of one focused beam (Jpn. J. Appl. Phis., 26 (1987), suppl. 26-4, pp. 61-66).
In the 1-beam overwritable phase-change recording medium, the time required for writing information can be shortened. It has a further merit that a drive can be constituted simply and inexpensively since the medium requires no magnetic fields.
Further a write-once-type phase-change medium can also be obtained by substantially the same material and layer constitution as those of the rewritable-type medium by changing the composition of the recording layer from that of the reversible phase-change type recording layer.
In this case, information can be recorded and stored for a longer period of time since the medium has no reversibility and the information can be stored, in principle, substantially permanently.
In a case of using the phase-change medium as the write-once-type medium, different from an ablation-type since no raising called as a rim is not caused to the periphery of a bit, it has a merit of providing excellent signal quality, and, since no gap is required above the recording layer, there is no requirement for an air sandwich structure.
The requirements of the high capacity and the high density in the recording medium is inevitable of the times, imposed on recording media and recording apparatus for handling an enormous amount of video information or audio signals, and they have been ever-progressing keeping pace with the progress of digital modulation technique and data compression technique. These high capacity and density are also demanded in the above-mentioned phase-change optical recording medium.
As a concrete means for increasing the recording density, in the optical disk, there have been developed and utilized, for example, reduction of a focused beam diameter of irradiated light and shortening of recording mark length by shortening the wavelength of the optical source or making NA (Numerical Aperture) of lens high, MCAV (Modified Constant Angular Velocity) of increasing recording frequency toward an outer circumference under constant rotational frequency, thereby making the recording density constant from inner to outer circumferences, and a mark edge recording of carrying information to beginning and rear ends of a mark, and means for further increasing the density has been thought at present.
Further in the phase-change-type medium, since there is less deterioration caused by the reduction of optical resolution and a signal amplitude can be increased even in a case of recording at an identical track pitch (track pitch density) and a shortest bit length (longitudinal recording density), it has a merit capable of easily attaining increased density as compared with the magneto-optical medium.
In an optical disk capable of recording, guide grooves are previous engraved on a disk to form so-called tracks. Usually, information signals are recorded, read out or erased by condensing a laser beam on a land or in a guide groove.
In an optical disk, lands and grooves are formed alternately in a radial direction coaxially or spirally and a focused light is guided by utilizing a diffracted light from the portions. The system includes a push-pull tracking-servo system of utilizing a radial difference of an intensity of a reflected light from an optical disk, namely, utilizing a diffracted light from a land or groove detecting 0th and 1st diffracted light by two splitted detectors, thereof (I1-I2 signal), and a 3-beam system using three splitted optical beams arranged in parallel in a radial direction and guiding a focused light by the calculation of the intensity of the reflected light for each of beams at three detector positions, that is, a land and grooves on both sides thereof or a groove and lands on both sides thereof. Further, the radial movement in such an optical disk is conducted by a system of counting the number of tracks passed by a cross track signal (I1+I2) and approaching an aimed track. In a usual optical disk, since recording/reading-out is performed only to the lands or only to the grooves, the width of the land (or groove) used for recording is made wider usually by about twice compared with that of the groove (or land) not used for recording. For further increasing the capacity, a system of recording/reading-out in both of the land and the groove is also considered. The capacity of the optical disk is doubled by recording both in the land and the groove.
In ordinary optical disks marketed at present, usually, information signals are recorded to either one of the land or the groove, and the other of them serves only as a boundary for separating adjacent tracks to prevent intrusion of leaked signals.
If information can be recorded also in the boundary portion, for example, in the groove in the case of recording information on the land, or on the land in a case of recording information in the groove, the recording density is doubled and remarkable improvement can be expected for the recording capacity.
A method of recording information to both of the land and the groove are hereinafter simply referred to as "L&G recording".
L&G recording is proposed, for example, in Japanese Patent Publication (KOKOKU) 63-57859 and a special care has to be taken for reducing cross-talk in a case of using such L&G recording technique.
That is in the L&G recording described in Japanese Patent Publication (KOKOKU) 63-57859, since the distance between a row of recording marks in a certain track and a row of recording marks in a track adjacent therewith is one-half of a focused beam diameter, the focused beam diameter lies over the row of recording marks adjacent with the row of recording marks to be read out. Therefore, cross talk upon reading-out is increased to deteriorate the reading-out S/N.
For reducing the cross talk, there is proposed a method as described for example, in SPIE Vol. 1316, Optical Data Storage (1990), pp 35, of disposing a special optical system and a cross talk cancel circuit to an optical disk reading-out device, thereby reducing the cross talk. However, this method involves a disadvantage of further complicating the optical system and the signal processing system of the device.
As a method of reducing the cross talk with no additional provision of special optical system or signal processing circuit for reducing the cross talk upon reading-out, it has been proposed to make the width of the groove (guide groove) equal with that of the land and define the groove depth within a certain range corresponding to a wavelength of a reading-out light (Jpn. J. Appl. Phys. Vol. 32 (1993), pp 5324 5328).
This proposal shows, by calculation and based on experiment that cross talk is reduced under the condition that the lane width is equal with the groove width and the groove depth is from .lambda./7n to .lambda./5n (.lambda.: wavelength of reading-out light, n: refractive index of substrate).
This is disclosed also in Japanese Patent Application Laid-Open (KOKAI) 5-282705.
In the above-mentioned proposals, based on the premise that the groove width is equal with the land width, an effect of reducing the cross talk is shown by computer simulation, examples of actually manufacturing, evaluating disks are given, and their effectiveness is mentioned.
It has been reported that a high density 3 to 4 times the current density can be attained by L&G recording method using the phase-change medium in CD size (diameter: 120 nm) by combination with an optical head at 680 nm and 0.6 NA (numerical aperture of a condensing lens) available at present (Jpn. J. Appl. Phys., 32 (1993), pp 5324 5328). It is also said that high quality moving picture for not less than one hour can be recorded in combined use with image compression technique.
However as a result of the further present inventions' earnest study, it has been found that as the recording density is increased by restriction of a track pitch by narrowing the groove width while keeping the groove width-to-land width ratio at 1:1, characteristics are remarkably deteriorated on the lands in view of residue after erasure of a preceding mark or worsening jitter of the recording mark after repetitive overwriting, and on the other hand, that erasing characteristic or jitter is less worsened after repetitive recording overwriting in the groove even if the track pitch is narrowed.
Further according to the dependence of the CN ratio (carrier-to-noise ratio) and the cross talk on the groove depth described in the afore-mentioned report, although an effect of reducing the cross talk can be obtained by optimizing the groove depth, balance of the CN ratio is lost between the lands and the grooves.
In the L&G recording, it is not preferable in view of the signal quality of a disk that a difference is caused between the carrier level on the lands and the carrier level in the grooves, and as a result, the CN ratio for one of them is remarkably deteriorated. The difference thereof should be fallen within a specified range.
Since atoms can migrate in the phase-change optical disk upon recording and erasure, there is a problem of characteristic deterioration caused by repetitive recording and erasure.
Although the repetitive recording characteristic can be improved to some extent by optimizing, for example, the material for the recording layer and the protective layer, the layer constitution and conditions for preparing each of the layers, it is not yet sufficient. As the cause for the deterioration due to repetitive recording and erasure, there may be considered, for example, film deformation, material transfer in the recording film and segregation. It has not seen a reason why such phenomenon became conspicuous.
Also, there has been proposed for example, a sample servo method for guiding an optical beam by the arrangement of pits formed with unevenness without disposing the guide grooves, other than L & G recording.
Although a narrow track pitch can be achieved by the method, in a case of recording at a track pitch of not more than 1.0 .mu.m, an extremely small spot diameter of the focused beam should be obtained by using an optical system at a short wavelength and with a great NA, but it has been known that the focal depth is also reduced in such an optical system.
Specifically, for the wavelength .lambda. and the lens numerical aperture NA, there are the following relationship. EQU Beam spot diameter.varies..lambda./NA EQU Focal depth.varies..lambda./(NA)2
The focal depth is abruptly reduced as the spot diameter at a focal point is restricted. Accordingly, if the focal point is automatically adjusted to the surface of the recording layer, a margin of a focus servo system is extremely narrowed. At the same time, the coma aberration caused by the tilt of the substrate increases.
One of the solutions is to reduce the thickness of the substrate to not more than 1.2 mm of the prior art (T. Sugaya, et al., Jpn. J. Appl. Phys. 32, 5402 (1993)).
Further if the track pitch is narrowed to 1.0 .mu.m in the L&G recording, sample servo recording and land and groove recording, there is a problem that a slight deviation (offset) of the focus servo system as described above increases leak signal from adjacent tracks (cross talk).
It has become apparent recently that the focus offset is increased by astigmatism caused by vertical birefringence of a substrate (M. R. Latta, et al, Proceeding of SPIE, vol. 1663 (1992), pp 157).
That is since the focused beam has astigmatism, the focal position is separated into two points to provide a focal position in which a beam is restricted into an elongated shape along the direction of the track and a focal position in which the beam is restricted in an elongate shape in the direction perpendicular to the track.
Such astigmatism is particularly conspicuous in a case of using a linearly polarized beam.
It depends on the combination of individual drives or substance, to which of positions focusing is adjusted automatically. Further, it is not always adjusted to one of the focusing points but focusing may be conducted at an intermediate position.
If the beam at the focused position take such a position as it becomes elongate in a direction perpendicular to the track the cross talk from the adjacent track increases upon reading-out. Further, if such a beam shape is formed during recording, it may possibly erase an amorphous bit already recorded in the adjacent track.
This is because the temperature of the adjacent track is easily elevated by heating to a bottom portion of the focused beam if the track pitch is not more than 1.0 .mu.m and smaller than the spot diameter of the focused beam.
It tends to occur easily that a portion of the amorphous bit is crystallized and erased during repetitive recording although such phenomenon is not caused by the recording only for once.
The problem around the birefringence of the substrate described above has been considered, as a problem, in a magneto-optical medium for detecting a minute Kerr rotation angle (W. A. Challener, et al., Applied Optics, Vol. 26, (1987), pp 3974, or I. Prikeryl, Applied Optics, vol. 31, (1992), pp 1853).
A phase difference caused by birefringence of the substrate presents a problem because of the physical property of the magneto-optical medium of detecting a minute ellipticity from a linearly polarized light and it has been considered that this scarcely causes a problem in the phase-change medium for detecting the intensity of the reflected light.
Therefore, only the in-plane birefringence, is noted for instance and it has been emphasized as a merit of the phase-change medium that it suffers from no effect of the noises or signal intensity even if the in-plane birefringence exceeds 20.times.10.sup.-6.
Accordingly, it can be said that no appropriate counter-measure has substantially been taken in the phase-change medium.
However, the problem of the astigmatism exists, for example, in a case of using a linearly polarized beam of a semiconductor laser for ensuring compatibility with a magneto-optical medium, or in a case of saving circular polarization by using a .lambda./4 plate for simplifying the structure of an optical head and even the phase-change medium not detecting the polarized state tends to undergo the effect of the astigmatism by the presence of birefringence.
By the way, although the recording density can be increased more as the pitch of the groove or pit for tracking (track pitch) disposed on the substrate is smaller, there is a limit for narrowing the track pitch since there is a diffraction limit in the beam spot system.
Usually, the track pitch may be selected such that the amount of the cross talk is reduced to less than a predetermined level, but there is another problem to be considered in the phase-change medium.
This is a problem that the amorphous bit in the adjacent track is erased (recrystallized) upon repetitive overwriting to a certain track.
The reason is not always apparent but it is assumed that the temperature of the adjacent track is elevated by a weak laser beam at the bottom portion of an intensity distribution of the focused beam upon recording the adjacent track, so that the amorphous bit is heated to a temperature higher than the crystallization temperature.
While the heating time per once is within several hundreds nano seconds, it is recrystallized although gradually during repetitive heating.
For instance, the C/N ratio (carrier-to-noise ratio) of the adjacent track is reduced from 55 dB at the initial state to not more than 50 dB after 10,000 cycles of repetitive overwriting.
The problem is hereinafter is simply referred to as "cross erasure". In the phase-change medium, a care has to be taken to a minimum track pitch by cross erasure rather than the optical diffraction limit, but the limit is not always apparent.
In a case of performing L&G recording or sample servo recording, since no unevenness effective to shielding for heat conduction is present between the adjacent track, different from the recording only to one of the land or the groove of the prior art, the temperature of the adjacent track tends to be raised more easily by heat diffusion. Such cross erasure become a serious problem.
Accordingly, the substantial limit for the track density is restricted rather by the limit of thermal separation (cross erasure) than the optical resolution power, that is, signal leakage from the adjacent track (cross talk).
According to the study made by the present inventors, if L&G recording is conducted at a linear velocity of 3 m/s to a medium having grooves and lands each at 0.7 .mu.m in width, by using a semiconductor laser at a wavelength of 680 nm and an optical head of 0.55 NA, the carrier level of signals recorded in the adjacent land or groove was lowered by 3 to 5 dB after 1,000 times of overwriting.
However in usual recording media, repetitive recording for more than 100 times is often conducted only in a case of rewriting the file management or allocation information on the recording medium. That is, only a limited region disposed to the inner circumference or the outer circumference of a disk referred to as FAT in the DOS format or TOC in the CD format is rewritten frequently.
The frequently rewriting region is less than 1% of the entire recordable region.
There may be such a case as in UNIX in which file management or allocation information is physically dispersed but it may suffice to consider an average number of writing and there is scarcely a possibility that a specified region is rewritten over 10,000 times.
It is considered that the situation will not change also in feature formats that recording is conducted while distinguishing the file management or allocation region and the contents thereof, and rewriting is concentrated only to a physically distinguished narrow region.
That is the recording density for the entire medium is restricted a present by less than 1% frequently rewritable region.
As a result of an earnest study of the present inventors, it has been found that an optical recording medium for recording, erasing and reading-out information by irradiation of a laser beam, comprising a lower dielectric protective layer, a phase-change-type recording layer, an upper dielectric protective layer and a metal reflective layer deposited orderly on a transparent substrate formed with grooves in which the grooves are formed on the transparent substrate such that the groove depth (d) can satisfy the following relation (1) and the groove width (GW) and land width (LW) can satisfy the following relation (2): EQU .lambda./7n&lt;d&lt;.lambda./5n (1) EQU 0.1 .mu.m&lt;GW&lt;LW (2)
in which both of the grooves and the lands are used for the recording region can reduce the cross talk from adjacent tracks and has excellent respective overwriting characteristics in the lands. On the basis of the findings the present invention have accomplished.